New York City health officials issued a warning about the increase in transmitted bacterial illness spread by rat urine after 24 cases were reported in 2023, the most for any year. 

Only six cases of leptospirosis have been reported in the city so far this year but numbers were trending upward, the New York City Department of Health said. Cases in New York are largely associated with exposure to materials contaminated with rat urine from the Norway rat. 

The disease can cause fever, headache, chills, muscle aches, vomiting, diarrhea, cough, conjunctival suffusion, jaundice, and rash, Celia Quinn, the deputy commissioner for the Division of Disease Control, who issued an April 12 memo warning of the disease.  

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If not treated, kidney failure, meningitis, liver damage, and respiratory distress can occur, she said. 

“Transmission occurs through direct contact with infectious urine or urine contaminated water, soil, or food, entering the body through open wounds or mucous membranes,” the memo states. 

The Bronx had the most cases with 37, while Manhattan had 28, the memo said. Person-to-person transmission is rare, officials said. 

The Leptospira bacteria can die within minutes in the dry heat and freezing cold, Quinn said. 

“The cold winters of NYC likely limit the extent to which leptospires can survive in the environment,” she said. “However excessive rain and unseasonably warm temperatures, factors associated with climate change, may support the persistence of leptospires in more temperate areas like NYC.”

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More than half of the 24 cases reported last year were reported between June and October, a period when there was warmer and wetter climate with excessive rain and “unseasonably” warm days.

The number of cases raised alarms because only three per year were reported from 2001 to 2020. From 2021 to 2023, the city received 15 reports of leptospirosis.

Some 3 million rats call New York City home, according to a study by a pest company. 

In December, Mayor Eric Adams said rat infestation was partly responsible for many New Yorkers leaving the city. 

“Some people who have children and families decide they want to go to a place where their children can play outdoors, larger green spaces, you want to see animals — you don’t see animals except for rats in New York,” Adams said when asked about the plunging population data.

“So there’s a combination of things,” he added. “And we are getting rid of those rats, by the way.”